How to Reliably Check for Null and Empty Strings in JavaScript?

Checking for Null Values in JavaScript

In JavaScript, checking for null values can be tricky. Consider the following example:

<code class="js">if (pass == null || cpass == null || email == null || cemail == null || user == null) {
  alert("fill all columns");
  return false;
}</code>

Why doesn't this code work as expected?

The reason is that JavaScript's equality checks (== and !=) are type-coercible. This means that JavaScript will attempt to convert the values being compared to the same type before performing the comparison. In this case, null will be coerced to the empty string, which is considered a "falsy" value in JavaScript. As a result, all of the conditions in the above code will always evaluate to false, regardless of the actual values of the variables.

Checking for Falsy Values

To check for null values in JavaScript, it's more reliable to use strict equality checks (=== and !===). These checks do not perform type coercion and will only return true if the values being compared are of the same type and value.

<code class="js">if (pass === null || cpass === null || email === null || cemail === null || user === null) {
  alert("fill all columns");
  return false;
}</code>

Checking for Empty Strings

However, in this particular scenario, it sounds like you are actually trying to check for empty strings (""). In that case, you can use the following simplified code:

<code class="js">if (!pass || !cpass || !email || !cemail || !user) {
  alert("fill all columns");
  return false;
}</code>

This code will check for empty strings, null, undefined, false, and the numbers 0 and NaN. Note that for numerical values, it is preferable to use strict equality checks (num === 0 or num !== -1) to avoid unexpected false positives.

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