Final answer:
To check for null or undefined in JavaScript, perform a strict equality check using === for exact type matching, or a loose equality check using == to handle both values with one check.
Step-by-step explanation:
In JavaScript, you can check if a variable is null or undefined by using a strict equality check or the loose equality check. A strict equality check (using ===) will only return true if the variable type and value are an exact match to either null or undefined. It's common to use the typeof operator when specifically checking for undefined. Here is an example:
if (variable === null) {
// Code to handle null
}
if (typeof variable === 'undefined') {
// Code to handle undefined
}
For a loose equality check (using ==), which can check for both null and undefined at the same time since they are loosely equal, you would write:
if (variable == null) {
// Code to handle both null and undefined
}
Note that using the loose equality allows you to compact your checks, but it's essential to be aware of what types are being compared to avoid unexpected results.