I am using WebGL...OpenGL ES2. This is my vertex shader. Question...

When I have branching in my shader like turning a 'directional light' or 'ambient light' on/off (in my case) by passing an argument to the shader (1.0 to turn it on or 0.0 to turn it off) am I better off using IF/THEN or math? What is the performance hit (if any) when using IF statements in a shader verus handling it with math? Like this...

var VSHADER_SOURCE =
	'attribute vec4 a_Position;\n' +
	'attribute vec4 a_Color;\n' +
	'attribute vec4 a_Normal;\n' +
	'uniform mat4 u_ProjMatrix;\n' +
	'uniform mat4 u_ViewMatrix;\n' +
	'uniform mat4 u_ModelMatrix;\n' +
	'uniform mat4 u_NormalMatrix;\n' +
	'uniform vec3 u_DirectionalLightColor;\n' +
	'uniform vec3 u_AmbientLightColor;\n' +
	'uniform vec3 u_DirectionalLightDirection;\n' +
	'uniform vec3 u_GlobalColor;\n' +
	'uniform float u_UseGlobalColor;\n' +
	'uniform float u_UseDirectionalLight;\n' +
	'uniform float u_UseAmbientLight;\n' +
	'varying vec4 v_Color;\n' +
	'void main() {\n' +
	'	gl_Position = u_ProjMatrix * u_ViewMatrix * u_ModelMatrix * a_Position;\n' +
	'	vec3 normal = normalize(vec3(u_NormalMatrix * a_Normal));\n' +
	'	float nDotL = max(dot(u_DirectionalLightDirection, normal), 0.0);\n' +
	'	vec3 colorToUse = u_GlobalColor + (vec3(a_Color) * u_UseGlobalColor) + (-u_GlobalColor * u_UseGlobalColor);\n' +
	'	vec3 diffuse = colorToUse + (u_DirectionalLightColor * nDotL * colorToUse * u_UseDirectionalLight) + (-colorToUse * u_UseDirectionalLight);\n' +
	'	vec3 ambient = u_UseAmbientLight * u_AmbientLightColor * colorToUse;\n' +
	'	v_Color = vec4(diffuse + ambient, a_Color.a);\n' +
	'}\n';

The end result is that setting u_UseAmbientLight or u_UseDirectionalLight to 1.0 turns them on...setting them to 0.0 turns them off.

Am I needlessly making my code harder to read or would there be an increase in performance by doing it this way (as opposed to an IF/THEN)?

Modern OpenGL implementations compile following :

if (cond) { x = A; } else { x = B; }

to something like this:

x = cond * A + (1 - cond) * B;

(actually they have much better opcodes to express this kind of code to hardware)

So I would recommend you to use actual if conditionals - GLSL compiler in some cases could optimize code much better than you manually introducing additional multiply operations.

Thanks for the information everyone! Will make the code a lot more readable.

Just premultiply light color with that intensity toggle at CPU.