chai-almost

Extends chai with assertions that allow numbers to be "almost" equal (as in, within some tolerance of one another). This is useful in particular when accounting for floating point rounding errors or other approximations.

Installation

npm install chai-almost --save-dev

Use

To use, import the module then invoke it with chai.use:

const chai = require('chai');
const chaiAlmost = require('chai-almost');

chai.use(chaiAlmost());

By default, chai-almost allows a tolerance of 1 x 10-6. This will let most rounding errors pass safely but will still reject most true inequalities. You may override this value by passing your desired tolerance as an argument to the module on invocation:

chai.use(chaiAlmost(0.1));

Assertions

The almost assertion may be used chainably with deep or shallow equality:

// shallow
expect(3.9999999).to.almost.equal(4);	        // passes
expect(3.9).to.almost.equal(4);			// fails
expect(4.0000001).to.be.almost(4);		// passes
expect(4.1).to.not.be.almost(4);		// passes

// deep
expect({ taco: 'pastor', num: 3.9999999 }).to.almost.eql({ taco: 'pastor', num: 4 }); // passes
expect([[1, 2, 2.9999999], 1.0000001]).to.be.deep.almost([[1, 2, 3], 1]);             // passes
expect({ taco: 'pastor', num: 3.9 }).to.not.almost.eql({ taco: 'pastor', num: 4 });   // passes

Equality checks on non-numbers are left unchanged:

expect('taco').to.almost.equal('taco');   // still passes
expect({ x: 5 }).to.be.almost({ x: 5 });  // still fails (shallow equality)
expect(['tacos', 2, 3]).to.be.deep.almost(['burritos', 2, 2.9999999]); // still fails

Specific tolerance

A single-instance custom tolerance for shallow or deep equality may be passed in as the second argument to almost:

expect(10).to.be.almost(15, 10)                   // passes
expect([4, 2, 5]).to.be.deep.almost([3, 4, 7], 3) // passes