How Can I Get Result of Math Operation in WebGL Shader

Question

There is a slow math operation. I don't want to do this by using javascript, I want to do in WebGL Shaders, but I need to get the value result of this operation.

        var VertexShaderCode = `
        precision lowp float;
        uniform float value;
        float result;
        void main(void) {
            result = ........ // math operation
            // return result
            gl_Position = vec4(0, 0, 0, 1);
        }
        `

        var vertexShader = gl.createShader(gl.VERTEX_SHADER)
        gl.shaderSource(vertexShader, VertexShaderCode)
        gl.compileShader(vertexShader)

        var FragmentShaderCode = `precision lowp float;
        void main() {
                gl_FragColor= vec4(0);
        }`

        var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER)
        gl.shaderSource(fragmentShader, FragmentShaderCode)
        gl.compileShader(fragmentShader)
        var shaderProgram = gl.createProgram()
        gl.attachShader(shaderProgram, vertexShader)
        gl.attachShader(shaderProgram, fragmentShader)
        gl.linkProgram(shaderProgram)
        return shaderProgram

How can I do this?