gl_FragCoord.x, y, z being 1 for all pixels and w being the depth

I'm using THREE.js with shader. I'm trying to get the zbuffer information.

Vertex shader:

// switch on high precision floats
#ifdef GL_ES
precision highp float;
#endif

void main()
{
    gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
}

Fragment shader:

#ifdef GL_ES
precision highp float;
#endif

void main()
{
    gl_FragColor = vec4(gl_FragCoord.x, gl_FragCoord.y, gl_FragCoord.z, 1.0);
}

There are objects with different position in the scene, thus the values in zbuffer should vary.

It's strange that gl_FragCoord.x, gl_FragCoord.y and gl_FragCoord.z seems to be 1.0 for all fragments, while gl_FragCoord.w seems to vary for different fragments.

If I use gl_FragCoord.w:

#ifdef GL_ES
precision highp float;
#endif

void main()
{
    float zbuffer = gl_FragCoord.w * 500.0;
    gl_FragColor = vec4(zbuffer, zbuffer, zbuffer, 1.0);
}

It seems to be the zbuffer image:

So why would gl_FragCoord.w be representing depth information while gl_FragCoord.z is always 1.0 for all fragments?