Manipulating chances with JS when using Math.random ()?

The algorithm below is made up of two parts:

• numTipo is generated: its value will be 0 if the return of a random generation of 1 to 10 is greater than 8, or 1 negative case; and
• numFinal simply contains a value between 2 and 100 that will always be , where numTipo is later added.

Adding an odd value to an even value will always generate an odd value. Any other addition case (Par + Par, Odd + Odd) generates an even number.

This algorithm forces the generation of values whose average distribution is 70% odd and 30% even.

function geraNum() {

  var numTipo = Math.floor((Math.random() * 10) + 1) > 7 ? 0 : 1;
  var numFinal = Math.floor((Math.random() * 50) + 1) *2 + numTipo;

  return numFinal;
}

function geraSerie() {

    var contaPar = 0;
    var contaImpar = 0;

    for (i = 0; i < 1000; i++) { 
        
        if (geraNum() % 2 == 0) {
            contaPar++;
        } else {
            contaImpar++;
        }
    }
    
    console.log("Pares: " + contaPar + ", Ímpares: " + contaImpar);    
}

geraSerie();
geraSerie();
geraSerie();
geraSerie();
geraSerie();

The name itself already says ramdom() , that is, it returns a random number. If you want to change the chances of the returned number to be more even or odd, you can create a simple function to check the return of the code:

let myNumber = Math.floor((Math.random() * 10) + 1);
if (myNumber % 2 == 0) { 
  // par = faça alguma coisa
} else {
  // ímpar = faça alguma coisa
}

But since it's a random return, only with if's can you manipulate the result.

Another way is for you to create your own function (Ex: Math.myRandomFunction() ) and in it you create your own logic, storing information and forcing the return to be more even or odd.

It would be more interesting to tell you which problem exactly you want to solve. But according to your example, you can first make a raffle to determine if the number you want is even or odd. Then, back home case, call a function that generates a number only in the pattern you want. For example:

if (x > 7) {
    return gerarNumeroPar();
} else {
    return gerarNumeroImpar();
} 

This function can be done by selecting from a previously generated list or even drawing successively until it hits what you need to generate.

I do not quite understand, but if the goal is to be more conducive to a result, you can do it this way:

if(valor > 7){
    // 30% de chance de cair aqui 
}else{
    // 70% de chance de cair aqui 
}