javascript hoisting: what would be hoisted first — variable or function?

问题

Recently I was confused about javascript hoisting behavior and now I got stuck with that.

So, there are two examples.

var alpha = 'alpha';

var beta = 'beta';

f(); //beta


var f = function f1() {
    console.log(beta);
};

function f() {
    console.log(alpha);
}

f(); // alpha

The first one is working as expected, because the function declaration overwrite our variable f (with value "undefined") when Javascript is set up the Lexical Environment.

But the second gives me a push, that I does not understand something.

var alpha = 'alpha';

var beta = 'beta';

f();  // - alpha


function f() {
    console.log(alpha);
}

var f = function f1() {
    console.log(beta);
};


f(); // beta

Why the variable f did'nt hoisted to the top of out code, and overwrite our hoisted function before? Why in "first f" call I recieve "alpha". I think, that it must be an error like: "f is not a function", because in first f call I excepect, that the f would be "undefined".

回答1:

Given var foo = something, only the variable declaration is hoisted.

This means that var foo is hoisted but the foo = something will run in the reading order.

Given function foo() {}, the variable declaration and function assignment are both hoisted. This creates the variable foo and gives it the value of the function.

If you have both of the above, then you declare the variable twice (for no extra effect) and assign it the value of the function (as that is the only assignment).

So in your second example…

The function declaration is hoisted, so f(); // - first f calls that.

The assignment of the function expression is not hoisted, but f(); // ** - second f appears after it in normal reading order, so the second call to foo() hits that.

回答2:

technically, the two snippets are the same

something like

function f() {
    console.log(alpha);
}
var alpha = 'alpha';
var beta = 'beta';
var f; // ignored because f is already defined
f();  // - first f
f = function f1() {
    console.log(beta);
};
f(); // ** - second f

or perhaps - though, I'm pretty sure function are "hoisted" first

var alpha = 'alpha';
var beta = 'beta';
var f;
function f() {
    console.log(alpha);
}
f();  // - first f
f = function f1() {
    console.log(beta);
};
f(); // ** - second f

来源：https://stackoverflow.com/questions/49899901/javascript-hoisting-what-would-be-hoisted-first-variable-or-function