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Truthy and Falsy: When All is Not Equal in JavaScript

    Craig Buckler
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    Comparing two things for equality can often trip up the unwary JavaScript developer, as the language has several quirks we need to be aware of.

    In this article, we’ll look at why that is, exploring both the double and triple equals operators, as well as the concept of truthy and falsy values in JavaScript. By the time you’ve finished reading, you’ll understand how JavaScript makes its comparisons, as well as how truthy and falsy values can help you write cleaner code.

    Truthy and Falsy: When All is Not Equal in JavaScript

    Typing in JavaScript

    JavaScript variables are loosely/dynamically typed and the language doesn’t care how a value is declared or changed:

    let x;
    x = 1;   // x is a number
    x = '1'; // x is a string
    x = [1]; // x is an array
    

    Seemingly different values equate to true when compared with == (loose or abstract equality) because JavaScript (effectively) converts each to a string representation before comparison:

    // all true
    1 == '1';
    1 == [1];
    '1' == [1];
    

    Truthy and Falsy: When All is Not Equal in JavaScript

    A more obvious false result occurs when comparing with === (strict equality) because the type is considered:

    // all false
    1 === '1';
    1 === [1];
    '1' === [1];
    

    Internally, JavaScript sets a value to one of seven primitive data types:

    • Undefined (a variable with no defined value)
    • Null (a single null value)
    • Boolean (a true or false value)
    • Number (this includes Infinity and NaN — not a number!)
    • BigInt (an integer value larger than 2^53 – 1)
    • String (textual data)
    • Symbol (a unique and immutable primitive new to ES6/2015)

    Everything else is an Object — including arrays.

    Truthy and Falsy: When All is Not Equal in JavaScript

    Truthy and Falsy Values in JavaScript

    As well as a type, each value also has an inherent Boolean value, generally known as either truthy or falsy. Some of the rules are a little bizarre, so understanding the concepts and effect on comparison helps when debugging JavaScript applications.

    The following values are always falsy:

    • false
    • 0 (zero)
    • -0 (minus zero)
    • 0n (BigInt zero)
    • '', "", `` (empty string)
    • null
    • undefined
    • NaN

    Everything else is truthy. That includes:

    • '0' (a string containing a single zero)
    • 'false' (a string containing the text “false”)
    • [] (an empty array)
    • {} (an empty object)
    • function(){} (an “empty” function)

    A single value can therefore be used within conditions. For example:

    if (value) {
      // value is truthy
    }
    else {
      // value is falsy
      // it could be false, 0, '', null, undefined or NaN
    }
    

    document.all

    You might also see document.all listed as a falsy value. This returns an HTMLAllCollection which contains a list of all of a document’s elements. And while this evaluates to false in a Boolean context, it’s a deprecated feature and MDN advises against its use.

    Truthy and Falsy: When All is Not Equal in JavaScript

    Loose Equality Comparisons with ==

    Unexpected situations can occur when comparing truthy and falsy values using the == loose equality:

    == true false 0 '' null undefined NaN Infinity [] {}
    true true false false false false false false false false false
    false false true true true false false false false true false
    0 false true true true false false false false true false
    '' false true true true false false false false true false
    null false false false false true true false false false false
    undefined false false false false true true false false false false
    NaN false false false false false false false false false false
    Infinity false false false false false false false true false false
    [] false true true true false false false false false false
    {} false false false false false false false false false false

    The rules:

    • false, zero and empty strings are all equivalent.
    • null and undefined are equivalent to themselves and each other but nothing else.
    • NaN is not equivalent to anything — including another NaN!.
    • Infinity is truthy — but cannot be compared to true or false!.
    • An empty array is truthy — yet comparing with true is false and comparing with false is true?!.

    Examples:

    // all true
    false == 0;
    0 == '';
    null == undefined;
    [] == false;
    !![0] == true;
    
    // all false
    false == null;
    NaN == NaN;
    Infinity == true;
    [] == true;
    [0] == true;
    

    Strict Equality Comparisons with ===

    The situation is clearer when using a strict comparison because the value types must match:

    === true false 0 '' null undefined NaN Infinity [] {}
    true true false false false false false false false false false
    false false true false false false false false false false false
    0 false false true false false false false false false false
    '' false false false true false false false false false false
    null false false false false true false false false false false
    undefined false false false false false true false false false false
    NaN false false false false false false false false false false
    Infinity false false false false false false false true false false
    [] false false false false false false false false false false
    {} false false false false false false false false false false

    The only exception is NaN, which remains stubbornly inequivalent to everything.

    Recommendations for Working with Truthy or Falsy Values

    Truthy and falsy values can catch out the most experienced developers. Those new to programming or migrating from other languages have no chance! Fortunately, there are three simple steps for catching the most difficult-to-spot errors when handling truthy and falsy variables. Let’s look at each in turn.

    1. Avoid direct comparisons

    It’s rarely necessary to compare two truthy and falsy values when a single value will always equate to true or false:

    // instead of
    if (x == false) // ...
    // runs if x is false, 0, '', or []
    
    // use
    if (!x) // ...
    // runs if x is false, 0, '', NaN, null or undefined
    

    2. Use === strict equality

    Use a === strict equality (or !== strict inequality) comparisons to compare values and avoid type conversion issues:

    // instead of
    if (x == y) // ...
    // runs if x and y are both truthy or both falsy
    // e.g. x = null and y = undefined
    
    // use
    if (x === y) // ...
    // runs if x and y are identical...
    // except when both are NaN
    

    3. Convert to real Boolean values where necessary

    You can convert any value to a real Boolean value in JavaScript using either the Boolean constructor, or a double-negative !!. This will allow you to be absolutely certain a false is generated only by false, 0, "", null, undefined and NaN:

    // instead of
    if (x === y) // ...
    // runs if x and y are identical...
    // except when both are NaN
    
    // use
    if (Boolean(x) === Boolean(y)) // ...
    // or
    if (!!x === !!y) // ...
    // runs if x and y are identical...
    // including when either or both are NaN
    

    The Boolean constructor returns true when passed a truthy value and returns false when passed a falsy value. This could be useful when combined with an iteration method. For example:

    const truthy_values = [
      false,
      0,
      ``,
      '',
      "",
      null,
      undefined,
      NaN,
      '0',
      'false',
      [],
      {},
      function() {}
    ].filter(Boolean);
    
    // Filter out falsy values and log remaining truthy values
    console.log(truthy_values);
    

    Conclusion

    Truthy and falsy values allow you to write terse JavaScript conditions and ternary operators. However, always consider the edge cases. A rogue empty array or NaN variable could lead to many hours of debugging grief!

    Do you need help with anything we’ve covered here? Why not head over to our JavaScript forum and ask a question. We have a team of friendly experts ready to help out.