Check if property exists in Javascript

Quickly check whether a property exists against an object.

There is more than one way to check whether a given object has a prop - the choice if often convenience rather than performance of the said method.

Let’s see the various methods by taking this example of a simple object.

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let apple = { color: "red", size: "small" };

Method 1: Check the value of the prop

Check whether a property exists in an object by “getting” that prop.

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let apple = { color: "red", size: "small" };

// get prop and booleanify it!
console.log(!!apple["color"]); // true

// or you could simply make use of Javascript's truthy checks for non-empty strings.
// how to use the truthy check in a statement
if (apple["color"]) console.log("apple has color"); // yes, apple has color

/* output
true
apple has color
*/

This is easy to code and to read, but what happens if you use a property that has no value?

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console.log("weight: ", apple["weight"]); // undefined
if (apple["weight"]) console.log("apple has weight"); // no show

The results are expected for an undefined prop, but let’s see the behaviour if the property exists but is falsy.

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let apple = { color: "red", size: "small", weight: "" };

console.log('apple["weight"]', apple["weight"]); // <blank>
if (apple["weight"]) console.log("apple has weight"); // no show

The unexpected results are a major turn-off.

Method 2: Check for prop but give it super powers

Avoid the confusing situation with prop checks using -

check for prop being undefined
optionally include typeof

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let apple = { color: "red", size: "small" };
console.log("1 weight: ", apple["weight"]); // undefined
if (apple["weight"]) console.log("1 apple has weight");
// not displayed

apple = { color: "red", size: "small", weight: "" };
console.log("2 weight: ", apple["weight"]); // <blank>
if (apple["weight"] != undefined) console.log("2 apple has weight");
// 2 apple has weight

You could make it even more clear by using typeof.

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if (typeof apple["weight"] != undefined) console.log("2 apple has weight"); // apple has weight

Method 3: Use `hasOwnProperty`

There is an alternate method that may be preferable in most cases - use hasOwnProperty method against the object.

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let apple = { color: "red", size: "small" };

console.log("apple has color:", apple.hasOwnProperty("color")); // true
console.log("apple has weight:", apple.hasOwnProperty("weight")); // false

Note that you could have made this more succinct/easy to remember using in, but that may have inconsistent results -

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console.log("apple has constructor?", "constructor" in apple); // true

The above code evaluates to true because Javascript internally tracks props like constructor (“non-enumerable props”) for all objects.

Summary

We saw three ways of checking for props in an object.