Improved understanding of js variables and deep understanding of js Variables

Improved understanding of js variables and deep understanding of js Variables

The Function Definition of JavaScript has a feature. It first scans the statements of the entire function body and "promotes" all declared variables to the top of the function:

'Use strict '; function foo () {var x = 'hello,' + y; alert (x); var y = 'bob';} foo ();

Although it is in strict mode, the statement var x = 'hello, '+ y; does not report an error because the variable y is declared later. However, alert displays Hello and undefined, indicating that the value of variable y is undefined. This is because the JavaScript engine automatically promotes the declaration of variable y, but does not increase the value assignment of variable y.

For the above foo () function, the JavaScript engine sees the code equivalent:

Function foo () {var y; // declare var x = 'hello, '+ y; alert (x); y = 'bob ';}

Because of this weird "feature" of JavaScript, when we define variables in a function, strictly abide by the rule "declare all variables in the function first. The most common practice is to use a var statement to declare all the variables used in the function:

Function foo () {var x = 1, // x is initialized to 1 y = x + 1, // y is initialized to 2 z, I; // The z and I are undefined // other statements: for (I = 0; I <100; I ++ ){...}}

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