Note: This feature is available in Web Workers.
The read-only WebGLShaderPrecisionFormat.precision property returns the number of bits of precision that can be represented.
For integer formats this value is always 0.
const canvas = document.getElementById("canvas");
const gl = canvas.getContext("webgl");
gl.getShaderPrecisionFormat(gl.VERTEX_SHADER, gl.MEDIUM_FLOAT).precision; // 23
gl.getShaderPrecisionFormat(gl.FRAGMENT_SHADER, gl.LOW_INT).precision; // 0