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CHAPTER 4 Expressions and Operators


Expressions and Operators

An expression is a phrase of JavaScript that a JavaScript interpreter can evaluate to produce a value. A constant embedded literally in your program is a very simple kind of expression. A variable name is also a simple expression that evaluates to whatever value has been assigned to that variable. Complex expressions are built from simpler expressions. An array access expression, for example, consists of one expression that evaluates to an array followed by an open square bracket, an expression that evaluates to an integer, and a close square bracket. This new, more complex expression evaluates to the value stored at the specified index of the specified array. Similarly, a function invocation expression consists of one expression that evaluates to a function object and zero or more additional expressions that are used as the arguments to the function.

The most common way to build a complex expression out of simpler expressions is with an operator. An operator combines the values of its operands (usually two of them) in some way and evaluates to a new value. The multiplication operator * is a simple example. The expression x * yevaluates to the product of the values of the expressions x and y. For simplicity, we sometimes say that an operator returns a value rather than “evaluates to” a value.

This chapter documents all of JavaScript’s operators, and it also explains expressions (such as array indexing and function invocation) that do not use operators. If you already know another programming language that uses C-style syntax, you’ll find that the syntax of most of JavaScript’s expressions and operators is already familiar to you.

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