If statement and variable hoisting

问题

I am learning about variable hoisting in JavaScript, and found this behavior weird:

var x = 0;

(function () {
    //Variable declaration from the if statement is hoisted:
    //var x; //undefined

    console.log(x); //undefined

    if (x === undefined) {
        var x = 1; //This statement jumps to the top of the function in form of a variable declaration, and makes the condition become true.
    }
}());

Is it correct, that in this case, the statement makes the condition true so it can be executed?

回答1:

Hoisting hoists only the declaration, but not the assignment. Your code is equivalent to:

var x = 0;

(function () {
    //Variable declaration from the if statement is hoisted:
    var x;

    console.log(x); //undefined

    if (x === undefined) {
        x = 1;
    }
}());

The if statement's condition expression evaluates to true and x = 1 is reached.

回答2:

By the way ,if you declare a variables in a if statement ,No matter the if condition pass or not pass,the declaration is always hosited.For example:

console.log(a); //undefined , not respond ReferenceError ,it has been hoisted
if(true){
  var a=1;
}
console.log(b); //same as above
if(false){
  var b=1;
}

来源：https://stackoverflow.com/questions/34149693/if-statement-and-variable-hoisting