JavaScript变量

【JavaScript变量】

Plus operator

An alternative method of retrieving a number from a string is with the + operator.

"1.1" + "1.1" = "1.11.1"
(+"1.1") + (+"1.1") = 2.2 // Note: the parentheses are added for clarity, not required.

Declaring variables

You can declare a variable in two ways:

With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables.
By simply assigning it a value. For example, x = 42. This always declares a global variable and generates a strict JavaScript warning. You shouldn't use this variant.

Evaluating variables

A variable declared using the var statement with no initial value specified has the value undefined.

An attempt to access an undeclared variable will result in a ReferenceError exception being thrown:

view plain print ?

var a;
console.log("The value of a is " + a); // prints "The value of a is undefined"
console.log("The value of b is " + b); // throws ReferenceError exception

You can use undefined to determine whether a variable has a value. In the following code, the variable input is not assigned a value, and the if statement evaluates to true.

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var input;
if(input === undefined){
doThis();
} else {
doThat();
}