Python partial equivalent in Javascript / jQuery

Question

What is the equivalent for Pythons functools.partial in Javascript or jQuery ?

Answer 1

Something like this perhaps. It is a little bit tricky as javascript doesn't have named arguments like python, but this function comes pretty close.

function partial() {
  var args = Array.prototype.slice.call(arguments);
  var fn = args.shift();
  return function() {
    var nextArgs = Array.prototype.slice.call(arguments);
    // replace null values with new arguments
    args.forEach(function(val, i) {
      if (val === null && nextArgs.length) {
        args[i] = nextArgs.shift();
      }
    });
    // if we have more supplied arguments than null values
    // then append to argument list
    if (nextArgs.length) {
      nextArgs.forEach(function(val) {
        args.push(val);
      });
    }
    return fn.apply(fn, args);
  }
}

// set null where you want to supply your own arguments
var hex2int = partial(parseInt, null, 16);
document.write('<pre>');
document.write('hex2int("ff") = ' + hex2int("ff") + '\n');
document.write('parseInt("ff", 16) = ' + parseInt("ff", 16));
document.write('</pre>');

Answer 2

ES6 solution

Here is a simple solution that works for ES6. However, since javascript doesn't support named arguments, you won't be able to skip arguments when creating a partial.

const partial = (func, ...args) => (...rest) => func(...args, ...rest);

Example

const greet = (greeting, person) => `${greeting}, ${person}!`;
const greet_hello = partial(greet, "Hello");

>>> greet_hello("Universe");
"Hello, Universe!"

Answer 3

To have the ES6 Solution also support classes:

const isClass = function (v) {
  // https://stackoverflow.com/a/30760236/2314626
  return typeof v === "function" && /^\s*class\s+/.test(v.toString());
};

const partial = (func, ...args) => {
  return (...rest) => {
    if (isClass(func)) {
      return new func(...args, ...rest);
    }
    return func(...args, ...rest);
  };
};

To use:

class Test {
  constructor(a, b, c) {
    this.a = a;
    this.b = b;
    this.c = c;
  }
}

const partialClass = partial(Test, "a");
const instance = partialClass(2, 3);
console.log(instance);

output:

? node tests/test-partial.js
Test { a: 'a', b: 2, c: 3 }

Answer 4

Here's an example using lodash:

const _ = require("lodash");

const greet = (greeting, person) => `${greeting}, ${person}!`;
const greet_hello = _.partial(greet, "Hello");

> greet_hello('John')
'Hello, John!'