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4.8.2 Unary Arithmetic Operators

4.8.2 Unary Arithmetic Operators


Unary operators modify the value of a single operand to produce a new value. In Java-Script, the unary operators all have high precedence and are all right-associative. The arithmetic unary operators described in this section (+, -, ++, and --) all convert their single operand to a number, if necessary. Note that the punctuation characters + and -are used as both unary and binary operators.

The unary arithmetic operators are the following:

Unary plus (+) The unary plus operator converts its operand to a number (or to NaN) and returns that converted value. When used with an operand that is already a number, it doesn’t do anything.

Unary minus (-) When -is used as a unary operator, it converts its operand to a number, if necessary, and then changes the sign of the result.

Increment (++) The ++ operator increments (i.e., adds 1 to) its single operand, which must be an lvalue (a variable, an element of an array, or a property of an object). The operator converts its operand to a number, adds 1 to that number, and assigns the incremented value back into the variable, element, or property.

The return value of the ++ operator depends on its position relative to the operand. When used before the operand, where it is known as the pre-increment operator, it increments the operand and evaluates to the incremented value of that operand. When used after the operand, where it is known as the post-increment operator, it increments its operand but evaluates to the unincremented value of that operand. Consider the difference between these two lines of code:

var i = 1, j = ++i; // i and j are both 2 var i = 1, j = i++; // i is 2, j is 1

Note that the expression ++xis not always the same as x=x+1. The ++operator never performs string concatenation: it always converts its operand to a number and increments it. If x is the string “1”, ++x is the number 2, but x+1 is the string “11”.

Also note that, because of JavaScript’s automatic semicolon insertion, you cannot insert a line break between the post-increment operator and the operand that precedes it. If you do so, JavaScript will treat the operand as a complete statement by itself and insert a semicolon before it.

This operator, in both its pre- and post-increment forms, is most commonly used to increment a counter that controls a for loop ( §5.5.3 ).

Decrement (--) The --operator expects an lvalue operand. It converts the value of the operand to a number, subtracts 1, and assigns the decremented value back to the operand. Like the ++ operator, the return value of --depends on its position relative to the operand. When used before the operand, it decrements and returns the decremented value. When used after the operand, it decrements the operand but returns the undecremented value. When used after its operand, no line break is allowed between the operand and the operator.

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