Re-defining Shader variables draws them all?

Question

I'm fairly new to shaders and came across the Book of Shaders website. Early on, this piece of code appears and surprisingly he didn't teach about how variables work yet so I can't get my head around the color variable. This code simultaneously displays a left to right fading background (black to white) and a green diagonal line.

So, essentially, you declare vec3 color to be vec3(y) which means all the 3 r,g,b values will be same throughout. I get why the fading background occurs because r, g, b stay equal and range between 0 and 1.

But coming from a JS and PHP background, normally if I change the value of a variable later, only the new value is accepted. So I was expecting that the lerping value out of color = (1.0-pct)*color+pct*vec3(0.0,1.0,0.0); would overwrite the previous vec3 color = vec3(y); and be considered for gl_FragColor function. But it appears both the versions of color are drawn: the fading BG and the green line. Is this how the shader code works, by drawing every definition of a variable?

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

// Plot a line on Y using a value between 0.0-1.0
float plot(vec2 st) {    
    return smoothstep(0.02, 0.0, abs(st.y - st.x));
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution;

    float y = st.x;

    vec3 color = vec3(y);

    // Plot a line
    float pct = plot(st);
    color = (1.0-pct)*color+pct*vec3(0.0,1.0,0.0);

    gl_FragColor = vec4(color,1.0);
}

Answer 1

First vec3 color = vec3(y); declares color and assigns the right to left black and white gradient to it. Then, color = (1.0-pct)*color+pct*vec3(0.0,1.0,0.0); assigns an new value to color which is a lerp between its old value (color), and its new value vec3(0.0,1.0,0.0) (green). It is equivalent to do :

color *= (1.0-pct);
color += pct*vec3(0.0,1.0,0.0);

The old value is overwritten but as the new definition uses this old value, you can still see the background gradient.