Expressions

The simplest expressions, known as primary expressions, are those that stand alone—they do not include any simpler expressions. Primary expressions in JavaScript are constant or literal values, certain language keywords, and variable references.

Literals are constant values that are embedded directly in your program. They look like these:

1.23         // A number literal
"hello"      // A string literal
/pattern/    // A regular expression literal

Reserved words like true, false, null, and this are primary expressions.

Finally, the third type of primary expression is the bare variable reference:

i      // Evaluates to the value of the variable i.
sum    // Evaluates to the value of the variable sum.

When any identifier appears by itself in a program, JavaScript assumes it is a variable and looks up its value. If no variable with that name exists, the expression evaluates to the undefined value. In the strict mode of ECMAScript 5, however, an attempt to evaluate a nonexistent variable throws a ReferenceError instead.