How do you check if a string is a number in javascript?

TL;DR

It depends largely on what you want to parse as a number.

Comparison Between Built-in Functions

As none of the existing sources satisfied my soul, I tried to figure out what actually was happening with these functions.

Three immediate answers to this question felt like:

1. !isNaN[input] [which gives the same output as +input === +input]
2. !isNaN[parseFloat[input]]
3. isFinite[input]

But are any of them correct in every scenario?

I tested these functions in several cases, and generated output as markdown. This is what it looks like:

So...which of them is "correct"?

Should be clear by now, it depends largely on what we need. For example, we may want to consider a null input as 0. In that case isFinite[] will work fine.

Again, perhaps we will take a little help from isNaN[] when 1010000000000 is needed to be considered a valid number [although the question remains—why would it be, and how would we handle that]!

Of course, we can manually exclude any of the scenarios.

Like in my case, I needed exactly the outputs of isFinite[], except for the null case, the empty string case, and the whitespace-only string case. Also I had no headache about really huge numbers. So my code looked like this:

/**
 * My necessity was met by the following code.
 */

if [input === null] {
    // Null input
} else if [input.trim[] === ''] {
    // Empty or whitespace-only string
} else if [isFinite[input]] {
    // Input is a number
} else {
    // Not a number
}

And also, this was my JavaScript to generate the table:

/**
 * Note: JavaScript does not print numeric separator inside a number.
 * In that single case, the markdown output was manually corrected.
 * Also, the comments were manually added later, of course.
 */

let inputs = [
    123, '123', 12.3, '12.3', '   12.3   ',
    1_000_000, '1_000_000',
    '0b11111111', '0o377', '0xFF',
    '', '    ',
    'abc', '12.34Ab!@#$',
    '10e100', '10e1000',
    null, undefined, Infinity];

let markdownOutput = `| \`input\` | \`!isNaN[input]\` or 
\`+input === +input\` | \`!isNaN[parseFloat[input]]\` | \`isFinite[input]\` | Comment |
| :---: | :---: | :---: | :---: | :--- |\n`;

for [let input of inputs] {
    let outputs = [];
    outputs.push[!isNaN[input]];
    outputs.push[!isNaN[parseFloat[input]]];
    outputs.push[isFinite[input]];

    if [typeof input === 'string'] {
        // Output with quotations
        console.log[`'${input}'`];
        markdownOutput += `| '${input}'`;
    } else {
        // Output without quotes
        console.log[input];
        markdownOutput += `| ${input}`;
    }

    for [let output of outputs] {
        console.log['\t' + output];
        if [output === true] {
            markdownOutput += ` | 
true
`;
            // markdownOutput += ` | ✔️`; // for stackoverflow
        } else {
            markdownOutput += ` | 
false
`;
            // markdownOutput += ` | ❌`; // for stackoverflow
        }
    }

    markdownOutput += ` ||\n`;
}

// Replace two or more whitespaces with $nbsp;
markdownOutput = markdownOutput.replaceAll[`  `, `  `];

// Print markdown to console
console.log[markdownOutput];

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