Operators

Operators are how we perform actions on variables and values. We’ve already seen two JavaScript operators, the = and the *.

The * operator performs mathematic multiplication. Simple enough, right?

The = equals operator is used for assignment — we first calculate the value on the right-hand side (source value) of the = and then put it into the variable that we specify on the left-hand side (target variable).

Warning: This may seem like a strange reverse order to specify assignment. Instead of a = 42, some might prefer to flip the order so the source value is on the left and the target variable is on the right, like 42 -> a (this is not valid JavaScript!). Unfortunately, the a = 42 ordered form, and similar variations, is quite prevalent in modern programming languages. If it feels unnatural, just spend some time rehearsing that ordering in your mind to get accustomed to it.

Consider:

a = 2;
b = a + 1;

Here, we assign the 2 value to the a variable. Then, we get the value of the a variable (still 2), add 1 to it resulting in the value 3, then store that value in the b variable.

While not technically an operator, you’ll need the keyword var in every program, as it’s the primary way you declare (aka create) variables (see “Variables”).

You should always declare the variable by name before you use it. But you only need to declare a variable once for each scope (see “Scope”); it can be used as many times after that as needed. For example:

var a = 20;
a = a + 1;
a = a * 2;
console.log( a );    // 42

Here are some of the most common operators in JavaScript:

• Assignment: = as in a = 2.
• Math: + (addition), - (subtraction), * (multiplication), and / (division), as in a * 3.
• Compound Assignment: +=, -=, *=, and /= are compound operators that combine a math operation with assignment, as in a += 2 (same as a = a + 2).
• Increment/Decrement: ++ (increment), -- (decrement), as in a++ (similar to a = a + 1).

• Object Property Access: . as in console.log().

  Objects are values that hold other values at specific named locations called properties. obj.a means an object value called obj with a property of the name a. Properties can alternatively be accessed as obj["a"]. See Chapter 2.

• Equality: == (loose-equals), === (strict-equals), != (loose not-equals), !== (strict not-equals), as in a == b.

  See “Values & Types” and Chapter 2.

• Comparison: < (less than), > (greater than), <= (less than or loose-equals), >= (greater than or loose-equals), as in a <= b.

  See “Values & Types” and Chapter 2.

• Logical: && (and), || (or), as in a || b that selects either a or b.

  These operators are used to express compound conditionals (see “Conditionals”), like if either a or b is true.

Note: For much more detail, and coverage of operators not mentioned here, see the Mozilla Developer Network (MDN)’s “Expressions and Operators” (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators).