Function is

is<Output>(value: unknown, condition: boolean): value is Output

Conditional type casting. You are responsible for ensuring that the condition implies the type casting.

Type Parameters

Output

Parameters

value: unknown
condition: boolean

Returns value is Output

Example: Spectrum

interface BaseSpectrum {
 dimension: number;
}
interface Spectrum1D extends BaseSpectrum {
 foo: number;
}
interface Spectrum2D extends BaseSpectrum {
 bar: number;
}

const spectrum: Spectrum = {...null};

if (is<Spectrum1D>(spectrum, spectrum.dimension === 1)) {
 // spectrum is Spectrum1D
 void spectrum.foo;
}
else if (is<Spectrum2D>(spectrum, spectrum.dimension === 2)) {
 // spectrum is Spectrum2D
 void spectrum.bar;
}

Example: Build spectrum check helper

Hide type checks with noisy syntax in dedicated helpers.

function isSpectrum1D(spectrum: Spectrum) {
  return is<Spectrum1D>(spectrum, spectrum.dimension === 1);
}

function isSpectrum2D(spectrum: Spectrum) {
  return is<Spectrum2D>(spectrum, spectrum.dimension === 1);
}