How to compare Enums in Typescript

Compare Enums in Typescript #

To compare enums, use dot notation to get the value for a specific enum property and compare it to another value, e.g. if (MyEnum.Small < 2) {}. The values for numeric enums, without provided initial value, are auto-incrementing integers, starting at 0.

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enum ShirtSizes {
  Small,
  Medium,
  Large,
}

const small = ShirtSizes.Small; // 👉️ 0
const medium = ShirtSizes.Medium; // 👉️ 1

if (small < medium) {
  console.log('✅ This runs');
}

if (small === 0) {
  console.log('✅ This runs');
}

const enumKey = ShirtSizes[ShirtSizes.Small];
console.log(enumKey); // 👉️ "Small"

if (enumKey === 'Small') {
  console.log('✅ This runs');
}

You can access specific values on an enum by using dot notation.

To compare the value against another, use one of the comparison operators, e.g. < (less than) > (greater than) or === (equals).

The if block is only ran if the condition is met.

The example shows a numeric enum, for which we haven't specified initial value for the value for the Small key is 0, Medium = 1 and Large = 2.

When using numeric enums, you can use reverse mappings to get the corresponding key based on a value.

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enum ShirtSizes {
  Small,
  Medium,
  Large,
}

const small = ShirtSizes.Small; // 👉️ 0
const medium = ShirtSizes.Medium; // 👉️ 1

const enumKey = ShirtSizes[ShirtSizes.Small];
console.log(enumKey); // 👉️ "Small"

const eKey = ShirtSizes[0];
console.log(eKey); // 👉️ Small

The examples above use reverse mappings to get the enum key, which corresponds to the value 0.

Here is an example of how to get an array of the keys and values of a string enum and check if a specific value is contained in the arrays.

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enum ShirtSizes {
  Small = 'SMALL',
  Medium = 'MEDIUM',
  Large = 'LARGE',
}

const keys = Object.keys(ShirtSizes);
console.log(keys); // 👉️ ['Small', 'Medium', 'Large']

if (keys.includes('Small')) {
  console.log('✅ This runs');
}

const values = Object.values(ShirtSizes);
console.log(values);

if (values.includes('SMALL' as unknown as ShirtSizes)) {
  console.log('✅ This runs');
}

Enums in TypeScript are real objects, so we are able to use the Object.keys() and Object.values() methods to get an array of the enum keys and values.

If working with numeric enums, the Object.keys() method returns an array containing the enum's keys and values, so you have to filter out the unnecessary values.

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enum ShirtSizes {
  Small,
  Medium,
  Large,
}

const keys = Object.keys(ShirtSizes).filter((v) => isNaN(Number(v)));
console.log(keys); // 👉️ ['Small', 'Medium', 'Large']

if (keys.includes('Small')) {
  console.log('✅ This runs');
}

const values = Object.values(ShirtSizes).filter((v) => !isNaN(Number(v)));
console.log(values); // 👉️ [0, 1, 2]

The concept with numeric enums is the same, however we had to add a call to the filter method to exclude the unnecessary values from the returned array.