Section 2.6. Expressions

2.6. Expressions

The simplest expressions are a literal value (such as a string or number), a variable, a built-in value (true, false, null, undefined, NaN, or Infinity), an invocation expression preceded by new, a refinement expression preceded by delete, an expression wrapped in parentheses, an expression preceded by a prefix operator, or an expression followed by:

• An infix operator and another expression

• The ? ternary operator followed by another expression, then by :, and then by yet another expression

• An invocation

• A refinement

The ? ternary operator takes three operands. If the first operand is truthy, it produces the value of the second operand. But if the first operand is falsy, it produces the value of the third operand.

The operators at the top of the operator precedence list in Table 2-1 have higher precedence. They bind the tightest. The operators at the bottom have the lowest precedence. Parentheses can be used to alter the normal precedence, so:

2 + 3 * 5 === 17
(2 + 3) * 5 === 25

Table 2-1. Operator precedence

. [] ( )	Refinement and invocation
delete new typeof + - !	Unary operators
* / %	Multiplication, division, modulo
+ -	Addition/concatenation, subtraction
>= <= > <	Inequality
=== !==	Equality
&&	Logical and
||	Logical or
?:	Ternary

The values produced by typeof are 'number', 'string', 'boolean', 'undefined', 'function', and 'object'. If the operand is an array or null, then the result is 'object', which is wrong. There will be more about typeof in Chapter 6 and Appendix A.

If the operand of ! is truthy, it produces false. Otherwise, it produces true.

The + operator adds or concatenates. If you want it to add, make sure both operands are numbers.

The / operator can produce a noninteger result even if both operands are integers.

The && operator produces the value of its first operand if the first operand is falsy. Otherwise, it produces the value of the second operand.

The || operator produces the value of its first operand if the first operand is truthy. Otherwise, it produces the value of the second operand.

Invocation causes the execution of a function value. The invocation operator is a pair of parentheses that follow the function value. The parentheses can contain arguments that will be delivered to the function. There will be much more about functions in Chapter 4.

A refinement is used to specify a property or element of an object or array. This will be described in detail in the next chapter.