I want to input the value from the range slider into get_projection function

Scale 0<input id="scaleSlider" type="range" min="0" max="100" step="10" value="50" />100

javascript code:

//var scale = 50 ;
var scale = document.getElementById("scaleSlider").value;
var proj_matrix = get_projection(scale, canvas.width / canvas.height, 1, 100);

The above code is that I tried the function only works with the 1st line and 2nd line it doesn't work. NOTE: the slider function is to scale a webGL 3D object.