My code works fine if I invoke gl.useProgram at the very beginning. On the other hand, If I never invoke gl.useProgram, nothing displayed and get two kind of waring in the console:

First:

WebGL: INVALID_OPERATION: uniformMatrix4fv: location is not from current program

Second:

WebGL: INVALID_OPERATION: drawElements: no valid shader program in use

The second warning makes sense to me, here is the signature for drawElements

drawElements(mode: GLenum, count: GLsizei, type: GLenum, offset: GLintptr):

When you issue a draw call, you need to know which program to run but the call signature has no hint for program, so a global program is necessary, so I put gl.useProgram just before gl.drawElements and everything works fine, but the first kind of warning persists which makes no sense to me! let see the signature for set Uniform

vertexAttrib4fv(index: GLuint, values: Iterable<GLfloat>): void;

the first is effectively an address in memory the program can refer to at the runtime and the address should be got from the program like

getUniformLocation(program: WebGLProgram, name: string): WebGLUniformLocation | null;

The program is already used to get the address, why the current global state is necessary when setting the uniform? Is there any kind of case I missed which does cause a problem?