Compare Values Safely in JavaScript: Equality, Objects and Dates

A practical approach

Step 1

Use === and !== by default, and enable a linter rule that flags == except for deliberate null checks.

Step 2

Compare dates by their numeric value, for example a.getTime() === b.getTime(), or compare ISO strings in a consistent time zone.

Step 3

Compare objects by the fields that define identity, such as an id, rather than by reference or by stringifying the whole object.

Step 4

Use Number.isNaN() to detect NaN, and Object.is() when you need to distinguish NaN or -0 explicitly.

Step 5

For deep equality in tests, use your test framework's deep-equal assertions instead of writing your own recursive comparison.

Worked example

A wishlist component checks whether a course is already saved with wishlist.includes(course). After data is refetched, the course objects are new instances, so includes always returns false and learners can save the same course twice. Comparing by identifier with wishlist.some(item => item.id === course.id) fixes the duplicates regardless of how the data was loaded.

A mistake to avoid

Do not compare objects with JSON.stringify(a) === JSON.stringify(b). Property order, undefined values and dates all affect the result, so equal data can compare as different and vice versa.

Try it yourself

Write five comparisons that surprise you in the browser console, such as [] == false and new Date(0) === new Date(0). For each, write the safe version and explain why it works.

Your implementation checklist

• Strict equality is the default and enforced by a linter
• Dates are compared by numeric value
• Objects are compared by identifying fields
• NaN is detected with Number.isNaN
• Tests use deep-equal assertions for structures