A question about scope in JS

This time I bring you a question about the scope of JS. What are the precautions for JS about the scope? The following is a practical case, let's take a look.

var a = 1;function foo () {    console.log(a);    var a = 2;    console.log(a);
}
foo();

Please write the output values ​​of the first console.log and the second console.log respectively.

JavascriptFunction Initialization sequence of variables

Pre-execution period

1. The function declaration is initialized and placed To the global variable (global context) window

2. The variable declared by var is placed in the global variable (global context) window, or the context where var is located, such as inside the function, and the initial value is undefined

Execution period

When the code is executed, variable assignment will occur. Variables declared without var are only created after the code is executed, and then placed in the global variable (global context) window

Javascript Context

Global Context: Window

Function context: Variables declared using var inside a function are local variables and will be garbage collected after leaving the scope of the function. Closures are not garbage collected.

That is, the variables declared by var will be placed in the corresponding context during the pre-execution period, and then during the execution period, the variables in the corresponding context will be assigned values.

Answer

undefined 2

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