Logical operators are most commonly used in JavaScript where the value of evaluated expressions are shared to come to a decision; these expressions may also be evaluated as not true, or in an either-or condition. As such, they’re often associated with if statements.

Logical AND: &&

Compares two conditions and returns a true or false summary. In the context of an if:

var x = 5;
var y = "Monday";
if (x > 3 && y == "Monday") {
    console.log("Success!");
}

The expressions are evaluated from left to right; both the expressions must return true in order for the Success! statement to be executed. If either is false, the if will be ignored, and “Success!” will never be written to the console. That is:

Logical AND Evaluations

Left	Right	Result
true	true	TRUE
true	false	FALSE
false	true	FALSE
false	false	FALSE

Logical OR: ||

In an OR comparison, any evaluation that is true will trigger conditional code:

var a = 16;
var b = "Slpadash";
if (a > 3 || b == "Exemplary") {
    console.log("Success!");
}

In the example above, the second evaluation will never be true, but the first one will be. Because an OR has been used, the console code will be run.

Logical NOT: !

Works as a “not” in a comparison statement:

var c = true;
var d = 1979;
if (c == true && d !== 1980) {
    console.log("Correct");
}

In the code above, Correct will be printed out. The comparison would read as “if c is true and d is not equal to 1980…”

! can also be used by itself:

var c = false;
var d = 1979;
if (!c || d <> 1980) {
    console.log("Correct");
}

The comparison would be read as “if c is false or d is greater or less than 1980”.

Conclusion

Logical operators are key to making complex decisions in JavaScript, and can be used in many different coding patterns, including ternary operators, which we’ll be looking at next.

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