Before learning about variable scope in JavaScript, we should clarify a few points:
•JavaScript's variable scope is based on its unique scope chain.
•JavaScript does not have block-level scope.
•Variables declared in a function are defined throughout the function.
1. JavaScript scope chain
First look at the following code:
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<script type="text/javascript"> var rain = 1; function rainman(){ var man = 2; function inner(){ var innerVar = 4; alert(rain); } inner(); //Call inner function } rainman(); //Call rainman function</script>
Observe the code alert(rain);. JavaScript first checks whether the variable rain is defined in the inner function. If it is defined, the rain variable in the inner function is used. If the rain variable is not defined in the inner function, JavaScript will continue to check whether the rain variable is defined in the rainman function. In this code, the rain variable is not defined in the rainman function body, so the JavaScript engine will continue to look up (global object) to see if rain is defined; in the global object, we have defined rain = 1, so the final result will pop up '1'.
Scope chain: When JavaScript needs to query a variable x, it will first search for the first object in the scope chain. If the first object does not define the If there is no definition, the search will continue, and so on.
The above code involves three scope chain objects, in order: inner, rainman, and window.
2. Within the function body, local variables have a higher priority than global variables with the same name.
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<script type="text/javascript"> var rain = 1; //Define the global variable rain function check(){ var rain = 100; //Define the local variable rain alert( rain ); //100 will pop up here } check (); alert( rain ); //1 will pop up here</script>
3. JavaScript does not have block-level scope.
This is also the part where JavaScript is more flexible than other languages.
Observe the code below carefully, you will find that the scopes of variables i, j, and k are the same, and they are global in the entire rain function body.
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<script type="text/javascript"> function rainman(){ // There are three local variables ijk in the rainman function body var i = 0; if ( 1 ) { var j = 0; for(var k = 0; k < 3; k++) { alert( k ); //Pop up 0 1 2 respectively } alert( k ); //Pop up 3 } alert( j ); //Pop up 0 }</script>
4. The variables declared in the function are defined throughout the function.
First observe this code:
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<script type="text/javascript"> function rain(){ var x = 1; function man(){ x = 100; } man(); //Call man alert( x ); //100 will pop up here } rain(); //Call rain</script>
The above code illustrates that the variable x can be used throughout the rain function body and can be reassigned. Due to this rule, "unbelievable" results will be produced, observe the code below.
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<script type="text/javascript"> var x = 1; function rain(){ alert( x ); //Pop up 'undefined' instead of 1 var x = 'rain-man'; alert( x ); / /pop 'rain-man' } rain();</script>
This is because the local variable x in the function rain is defined throughout the function body (var x= 'rain-man', declared), so the global variable x with the same name is hidden in the entire rain function body. The reason why 'undefined' pops up here is because when alert(x) is executed for the first time, the local variable x has not yet been initialized.
So the rain function above is equivalent to the following function:
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function rain(){ var x; alert( x ); x = 'rain-man'; alert( x );}
5. Variables defined without using the var keyword are global variables.
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<script type="text/javascript"> function rain(){ x = 100; //Global variable x is declared and assigned} rain(); alert( x ); //100 will pop up </script>
This is also a common mistake among JavaScript newbies, leaving many global variables unintentionally.
6. Global variables are all properties of the window object.
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<script type="text/javascript"> var x = 100; alert( window.x);//pop up 100 alert(x);</script>
Equivalent to the following code
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<script type="text/javascript"> window.x = 100; alert( window.x ); alert(x)</script>