Strange javascript scoping issue

问题:

var c = 1;

function myFunction(){
    c = 2;
    var c = 4;
    console.log(c);
}

console.log(c);
myFunction();
console.log(c);

Why is the last console.log spitting out 1? Here is how it is supposed to work in my brain:

var c = 1; // create global variable called 'c'

function myFunction(){
    c = 2; // assign value of global 'c' to 2
    var c = 4; // create a new variable called 'c' which has a new address in memory (right?)  with a value of 4
    console.log(c); // log the newly created 'c' variable (which has the value of 4)
}

console.log(c); //log the global c
myFunction(); //log the internal c
console.log(c); //log the updated global c

回答1:

In the scope where your console.log runs, only the global variable exists. And the function never touches the global variable.

The details:

var c = 1; // create global variable called 'c'

function myFunction(){
    // the global is unavailable here, as you declared a local c.
    // Its declaration is hoisted to the top of the function, so
    // c exists here, and its value is undefined.
    c = 2; // assign value of local 'c' to 2
    var c = 4; // change local c to 4
    console.log(c); // log local c (4)
}

console.log(c); //log the global c
myFunction(); //log the internal c
console.log(c); //log the global c (which never changed)

Due to the hoisting mentioned above, your function behaves as if the code was:

function myFunction(){
    var c; // declares a local c that shadows the global one
    c = 2;
    c = 4;
    console.log(c);
}

Some references about hoisting

• MDN page for the var statement
• MDN scope cheatsheet
• How are javascript variables "hoisted" in these examples from MDN
• Javascript hoisting explained
• JavaScript Scoping and Hoisting