228 lines
6.4 KiB
Text
228 lines
6.4 KiB
Text
## lodash/fp
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The `lodash/fp` module promotes a more
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[functional programming](https://en.wikipedia.org/wiki/Functional_programming)
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(FP) friendly style by exporting an instance of `lodash` with its methods wrapped
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to produce immutable auto-curried iteratee-first data-last methods.
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## Installation
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In a browser:
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```html
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<script src='https://cdn.jsdelivr.net/g/lodash@4(lodash.min.js+lodash.fp.min.js)'></script>
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<script>
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// Loading `lodash.fp.js` converts `_` to its fp variant.
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_.defaults({ 'a': 2, 'b': 2 })({ 'a': 1 });
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// ➜ { 'a': 1, 'b': 2 }
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// Use `noConflict` to restore the pre-fp variant.
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var fp = _.noConflict();
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_.defaults({ 'a': 1 }, { 'a': 2, 'b': 2 });
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// ➜ { 'a': 1, 'b': 2 }
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fp.defaults({ 'a': 2, 'b': 2 })({ 'a': 1 });
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// ➜ { 'a': 1, 'b': 2 }
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</script>
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```
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In Node.js:
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```js
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// Load the fp build.
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var fp = require('lodash/fp');
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// Load a method category.
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var object = require('lodash/fp/object');
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// Load a single method for smaller builds with browserify/rollup/webpack.
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var extend = require('lodash/fp/extend');
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```
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## Mapping
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Immutable auto-curried iteratee-first data-last methods sound great, but what
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does that really mean for each method? Below is a breakdown of the mapping used
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to convert each method.
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#### Capped Iteratee Arguments
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Iteratee arguments are capped to avoid gotchas with variadic iteratees.
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```js
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// The `lodash/map` iteratee receives three arguments:
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// (value, index|key, collection)
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_.map(['6', '8', '10'], parseInt);
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// ➜ [6, NaN, 2]
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// The `lodash/fp/map` iteratee is capped at one argument:
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// (value)
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fp.map(parseInt)(['6', '8', '10']);
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// ➜ [6, 8, 10]
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```
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Methods that cap iteratees to one argument:<br>
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<%= toFuncList(_.keys(_.pickBy(mapping.iterateeAry, _.partial(_.eq, _, 1)))) %>
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Methods that cap iteratees to two arguments:<br>
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<%= toFuncList(_.keys(_.pickBy(mapping.iterateeAry, _.partial(_.eq, _, 2)))) %>
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The iteratee of `mapKeys` is capped to one argument: `(key)`
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#### Fixed Arity
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Methods have fixed arities to support auto-currying.
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```js
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// `lodash/padStart` accepts an optional `chars` param.
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_.padStart('a', 3, '-')
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// ➜ '--a'
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// `lodash/fp/padStart` does not.
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fp.padStart(3)('a');
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// ➜ ' a'
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fp.padCharsStart('-')(3)('a');
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// ➜ '--a'
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```
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Methods with a fixed arity of one:<br>
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<%= toFuncList(_.difference(mapping.aryMethod[1], _.keys(mapping.skipFixed))) %>
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Methods with a fixed arity of two:<br>
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<%= toFuncList(_.difference(mapping.aryMethod[2], _.keys(mapping.skipFixed))) %>
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Methods with a fixed arity of three:<br>
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<%= toFuncList(_.difference(mapping.aryMethod[3], _.keys(mapping.skipFixed))) %>
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Methods with a fixed arity of four:<br>
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<%= toFuncList(_.difference(mapping.aryMethod[4], _.keys(mapping.skipFixed))) %>
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#### Rearranged Arguments
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Method arguments are rearranged to make composition easier.
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```js
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// `lodash/filter` is data-first iteratee-last:
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// (collection, iteratee)
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var compact = _.partial(_.filter, _, Boolean);
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compact(['a', null, 'c']);
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// ➜ ['a', 'c']
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// `lodash/fp/filter` is iteratee-first data-last:
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// (iteratee, collection)
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var compact = fp.filter(Boolean);
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compact(['a', null, 'c']);
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// ➜ ['a', 'c']
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```
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##### Most methods follow these rules
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A fixed arity of two has an argument order of:<br>
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<%= toArgOrder(mapping.aryRearg[2]) %>
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A fixed arity of three has an argument order of:<br>
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<%= toArgOrder(mapping.aryRearg[3]) %>
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A fixed arity of four has an argument order of:<br>
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<%= toArgOrder(mapping.aryRearg[4]) %>
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##### Exceptions to the rules
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Methods that accept an array as their last, second to last, or only argument:<br>
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<%= toFuncList(_.keys(mapping.methodSpread)) %>
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Methods with unchanged argument orders:<br>
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<%= toFuncList(_.keys(mapping.skipRearg)) %>
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Methods with custom argument orders:<br>
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<%= _.map(_.keys(mapping.methodRearg), methodName => {
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const orders = mapping.methodRearg[methodName];
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return ' * `_.' + methodName + '` has an order of ' + toArgOrder(orders);
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}).join('\n') %>
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The iteratee of `reduceRight` has an argument order of: `(b, a)`
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#### New Methods
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Not all variadic methods have corresponding new method variants. Feel free to
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[request](https://github.com/lodash/lodash/blob/master/.github/CONTRIBUTING.md#feature-requests)
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any additions.
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Methods created to accommodate Lodash’s variadic methods:<br>
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<%= toFuncList(_.keys(mapping.remap)) %>
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#### Aliases
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There are <%= _.size(mapping.aliasToReal) %> method aliases:<br>
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<%= _.map(_.keys(mapping.aliasToReal).sort(), alias => {
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const realName = mapping.aliasToReal[alias];
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return ' * `_.' + alias + '` is an alias of `_.' + realName + '`';
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}).join('\n') %>
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## Placeholders
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The placeholder argument, which defaults to `_`, may be used to fill in method
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arguments in a different order. Placeholders are filled by the first available
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arguments of the curried returned function.
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```js
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// The equivalent of `2 > 5`.
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_.gt(2)(5);
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// ➜ false
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// The equivalent of `_.gt(5, 2)` or `5 > 2`.
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_.gt(_, 2)(5);
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// ➜ true
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```
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## Chaining
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The `lodash/fp` module **does not** convert chain sequence methods. See
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[Izaak Schroeder’s article](https://medium.com/making-internets/why-using-chain-is-a-mistake-9bc1f80d51ba)
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on using functional composition as an alternative to method chaining.
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## Convert
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Although `lodash/fp` & its method modules come pre-converted, there are times
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when you may want to customize the conversion. That’s when the `convert` method
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comes in handy.
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```js
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// Every option is `true` by default.
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var _fp = fp.convert({
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// Specify capping iteratee arguments.
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'cap': true,
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// Specify currying.
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'curry': true,
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// Specify fixed arity.
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'fixed': true,
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// Specify immutable operations.
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'immutable': true,
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// Specify rearranging arguments.
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'rearg': true
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});
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// The `convert` method is available on each method too.
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var mapValuesWithKey = fp.mapValues.convert({ 'cap': false });
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// Here’s an example of disabling iteratee argument caps to access the `key` param.
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mapValuesWithKey(function(value, key) {
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return key == 'a' ? -1 : value;
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})({ 'a': 1, 'b': 1 });
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// => { 'a': -1, 'b': 1 }
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```
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Manual conversions are also possible with the `convert` module.
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```js
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var convert = require('lodash/fp/convert');
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// Convert by name.
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var assign = convert('assign', require('lodash.assign'));
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// Convert by object.
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var fp = convert({
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'assign': require('lodash.assign'),
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'chunk': require('lodash.chunk')
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});
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// Convert by `lodash` instance.
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var fp = convert(lodash.runInContext());
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```
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## Tooling
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Use [eslint-plugin-lodash-fp](https://www.npmjs.com/package/eslint-plugin-lodash-fp)
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to help use `lodash/fp` more efficiently.
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