The first time I know apply was when I met this code:Math.max.apply(null, [1, 2, 3, 4])As the mdn said,The apply() method calls a function with a given this value, and arguments provided as an array (or an array-like object).Note: While the syntax of this function is almost identical to that of call(), the fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.Actually, in case above, thisArg has no influence which means code below also works:Math.max.apply(undefined, [1, 2, 3, 4])Math.max.apply(Math, [1, 2, 3, 4])The only effect of apply in the code above is that it can pass the values in array to the function max. So, code above equalMath.max(1, 2, 3, 4)Why would I mention this? Because we don’t need this anymore because we already have … which works like:Math.max(…[1, 2, 3, 4])The reason that we still need apply and call is the thisArg. They can help us call some powerful methods.thisArg in apply and callI guess you might have seen this code:Array.prototype.slice.call({ length: 2 })function fn() { console.log(Array.prototype.slice.call(arguments))}fn(1, 2, 3, 4) //[1,2,3,4]Today, we don’t need this either because of Array.from. But I still want to talk about it for explanation. In the case above, call was used because we want to do something like:let obj = { length: 2 }obj.slice() //Uncaught TypeError: obj.slice is not a functionIt would cause error because slice was defined in Array.prototype. Only Array instance can call that method. But actually in the implementation of slice, it doesn’t need to be called by Array instance and there is a lot of methods like this. So, in this case, call or apply would let non Array instance call these methods which meansArray.prototype.slice.call({ length: 2 })//help you dolet obj = { length: 2 }obj.slice = Array.prototype.sliceobj.slice()And to help it easier to understand , you can remember it like:method.call(thisArg, …args)//works like in most casesthisArg.method = methodthisArg.method(…args)//for applymethod.apply(thisArg, args)//works like in most casesthisArg.method = methodthisArg.method(…args)Wasn’t that easy ?So, let get back to Math.max.apply({}, [1, 2, 3, 4]). You can remember it like:let thisArg = {}thisArg.max = Math.maxthisArg.max(…[1, 2, 3, 4])And more cases:Object.prototype.toString.call([]) //"[object Array]"//help you do thislet thisArg = []thisArg.toString = Object.prototype.toStringthisArg.toString() //"[object Array]"//while[].toString()//““Or;[’ sd ‘, 1, 3].map(Function.prototype.call, String.prototype.trim) //[‘sd’,‘1’,‘3’]//help you do;[’ sd ‘, 1, 3].map(function(…args) { return String.prototype.trim.call(…args)})//help you do;[’ sd ‘, 1, 3].map(function(…args) { let thisArg = args[0] thisArg.trim = String.prototype.trim return thisArg.trim(…args.slice(1)) //Uncaught TypeError: thisArg.trim is not a function})In the case above, it will got error because args[0] is Primitive values. You can’t call methods in Primitive values. But it can still help you understand.More in applyAs apply can accept an array-like object. So, what would happen if coding like:Array.apply(null, { length: 2 })Actually, it equalsArray.apply(null, [undefined, undefined])So, you can understand it like:let thisArg = {} //set null would get error in code below, also thisArg in above case is not importantthisArg.Array = ArraythisArg.Array(undefined, undefined)Hope it’s easier to understand apply and call.Original Post