回报如何在JS幕后运作

Question

I am new to JS and wanted to deeply understand how return in JS works. For example, here are some code examples: let num = function(){return 5} and function number(){return 5} console.log(number()) . The question is for example, in the second example Will number() which was passed to console.log() be replaced with the value that number() returns, that is, will console.log(number()) turn into console.log(5) behind the scenes. When we return something from a function then will that function be replaced with the value it returns. Is it what happens behind the scenes when we return a value from a function?

Answer 1

both assign a name to a function, but one of them perform at run time, the other at compile time (or at begin of running), that's why if you call number it's known even if it defined below the call, but for the other, it should first get assigned.

the return work the same in both, returning value 5. when you used () for calling the function.

for more information on JS structure i suggest You don't Know JS book. i'm not sure if it's allowed to suggest a book here, but it's free to read: You Don't Know JS

Update

If by any change i got you wrong, and you are looking for a way to pass the actual function:

When you are calling num() or number() inside the console.log() like this:

console.log(num()) 

It will process the 'num()' first, and then return the value which is '5', then that value, is passed to console.log(value) in here console.log(5) .

If you are looking for a way to pass the actual function/expression to the sub function, like what C# LINQ or Java Stream does, you need the secondary function to call the function it self. for example:

in here i wrote a function which filter out item of array, when user condition is true... Here condition is a function which passed to this function by user... and get called by secondary function...

function filter(list, condition) {
  let  result=[];
  for(let i = 0; i < list.length ; i++) {
    if(condition(list[i])) {
      result.push(list[i]);
    }
  }
  return result;
}

let condition = function(item) {
  return item.isOk;
}

and you call it like this:

filter(myArray, condition)

Note: i used condition instead of 'condition()', now we passed the actual function instead of the value returned by that function

Answer 2

In your first example you are assigning anonymous function yo a variable num . That is called function expression. But function has not been executed, so the num will remain a function/object. To get the value from the function you should execute that with the parentheses num() . Or if you want immediately return value and assign that to a variable you should do like this:

let num = (function () { return 5 })()

In your second example you first declare function and then in your console.log before showing on console you executes that number function and returns 5 and only after that your console.log will be executed and 5 will be passed as an argument.