Install npm install declaration-syntax-tester --save-dev Using with tape const test = require('tape');const {accept, reject} = require('declaration-syntax-tester')(); test('sample...', t => { // Write test case for accepted syntax of declaration files t.error(accept(` import sample from '../sample'; const x:number = sample.getNumber(); `), 'should getNumber() return number.'); // Write test case for rejected syntax of declaration files t.error(reject(` import sample from '../sample'; const x:string = sample.getNumber(); `), 'should return value of getNumber() can not assign to string.');}) A sample project typings-d3