Having a problem with rewriting invoke function inspired by underscore library

Question

I'm a beginner and trying to rewrite an underscore function _.invoke. I'm trying to make the function so it returns an array with the results of calling the method on each value in the collection.

_.invoke = function(collection, methodName) {
  var result = [];
  if (Array.isArray(collection)) {
    for (let i = 0; i < collection.length; i++) {
      methodName.call(collection[i])
      var value = collection[i][methodName]
      result.push(value)
    }
  }
  return result
}

I think my problem lays in this line:

methodName.call(collection[i]) - would like to invoke method on an object collection[i] but I would like to pass some arguments should they be included in the unit test.

I've tried so far utilizing the test : typeof(methodName) === "function" and writing a function that would test whether the method is a function.

Answer 1

You mean something like this?

const myArr = [
  { cons:function(args) { return args } },
  { cons:function(args) { return args["bla"] } },
]

const _ = {};
_.invoke = (collection, methodName, ...args) => !Array.isArray(collection) ? [] : collection
.filter(item => typeof item[methodName] === 'function')
.map(item => item[methodName].apply(item, args));

const res = _.invoke(myArr,"cons",{"bla":"hello"})
console.log(res)

Here is a more performant version if you need speed over a lot of entries

const myArr = [
  { cons: function(args) { return args; } },
  { cons: function(args) { return args["bla"]; } },
];

const _ = {};
_.invoke = (collection, methodName, ...args) => {
  const result = [];
  if (!Array.isArray(collection)) return result;

  for (let i = 0, len = collection.length; i < len; i++) {
    const item = collection[i];
    const method = item[methodName];

    if (typeof method === 'function') {
      result.push(method.apply(item, args));
    }
  }

  return result;
};

const res = _.invoke(myArr, "cons", {
  "bla": "hello"
});
console.log(res);

Answer 2

Here you can invoke with arguments.

_.invoke = function(collection, methodName, ...args) {
  if (!Array.isArray(collection)) {
     return [];
  }
  const out = []; 
  for(const item of collection){
    if(typeof item[methodName] === 'function')
      out.push(item[methodName].apply(item, args));
    }
  }
  return out;
}

To test that all items have a method:

const collection = [...];
const allHaveMethod = _.invoke(collection, 'method', 'arg1', 'arg2').length === collection.length;