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fftfreq

Return the Discrete Fourier Transform sample frequencies. The returned array contains the frequency bin centers in cycles per unit of the sample spacing. 
function fftfreq(n: number, d?: number): NDArray
NameTypeDefaultDescription
nnumber—Window length (number of samples).
dnumber1.0Sample spacing (inverse of the sampling rate).
Returns: NDArray — Float64 array of length n containing the sample frequencies. For even n, the layout is [0, 1, ..., n/2-1, -n/2, ..., -1] / (d*n). For odd n, it is [0, 1, ..., (n-1)/2, -(n-1)/2, ..., -1] / (d*n). 
import * as np from 'numpy-ts';

const freq = np.fft.fftfreq(5, 0.1);
// array([ 0.,  2.,  4., -4., -2.])

const freq8 = np.fft.fftfreq(8);
// array([ 0.   ,  0.125,  0.25 ,  0.375, -0.5  , -0.375, -0.25 , -0.125])

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rfftfreq

Return the Discrete Fourier Transform sample frequencies for use with rfft. Since rfft returns only the positive half of the spectrum, this function returns frequencies for bins 0, 1, ..., n//2. 
function rfftfreq(n: number, d?: number): NDArray
NameTypeDefaultDescription
nnumber—Window length (number of samples in the original signal, not the rfft output).
dnumber1.0Sample spacing.
Returns: NDArray — Float64 array of length n//2 + 1 containing the non-negative sample frequencies. 
import * as np from 'numpy-ts';

const freq = np.fft.rfftfreq(8);
// array([0.   , 0.125, 0.25 , 0.375, 0.5  ])

const freq_hz = np.fft.rfftfreq(1000, 1 / 44100);
// Frequency bins in Hz for a 1000-sample window at 44.1 kHz

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fftshift

Shift the zero-frequency component to the center of the spectrum. This is useful for visualizing the Fourier transform with the DC component in the middle. 
function fftshift(a: ArrayLike, axes?: number | number[]): NDArray
NameTypeDefaultDescription
aArrayLike—Input array.
axesnumber | number[]all axesAxes over which to shift.
Returns: NDArray — The shifted array. 
import * as np from 'numpy-ts';

const freq = np.fft.fftfreq(5, 0.1);
// array([ 0.,  2.,  4., -4., -2.])

const shifted = np.fft.fftshift(freq);
// array([-4., -2.,  0.,  2.,  4.])

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ifftshift

Inverse of fftshift. Undoes the centering performed by fftshift, moving the zero-frequency component back to the beginning. 
function ifftshift(a: ArrayLike, axes?: number | number[]): NDArray
NameTypeDefaultDescription
aArrayLike—Input array.
axesnumber | number[]all axesAxes over which to shift.
Returns: NDArray — The unshifted array. 
import * as np from 'numpy-ts';

const freq = np.fft.fftfreq(5, 0.1);
const shifted = np.fft.fftshift(freq);
const restored = np.fft.ifftshift(shifted);
// restored matches freq exactly

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Usage Pattern

A common workflow combines these utilities with the FFT functions: 
import * as np from 'numpy-ts';

// Generate a signal: 50 Hz sine wave sampled at 1000 Hz
const fs = 1000;           // Sampling frequency
const t = np.arange(0, 1, 1 / fs);  // 1 second of samples
const signal = np.sin(np.multiply(2 * Math.PI * 50, t));

// Compute FFT and corresponding frequencies
const spectrum = np.fft.fft(signal);
const freq = np.fft.fftfreq(signal.size, 1 / fs);

// Center the spectrum for visualization
const centered_spectrum = np.fft.fftshift(spectrum);
const centered_freq = np.fft.fftshift(freq);

// Or use rfft for real signals (more efficient)
const half_spectrum = np.fft.rfft(signal);
const half_freq = np.fft.rfftfreq(signal.size, 1 / fs);