my code only ever out puts "You win, rock beats scissors."

This is the code, now matter what I input as the input parameter in the play function, it alwats returns "You win, rock beats scissors."

    function computerPlay() {
        switch(Math.floor(Math.random() * 3) + 1) {
            case 1:
                return "rock";
                break;
            case 2:
                return "paper";
                break;
            case 3:
                return "scissors";
                break;
            default:
                return "error1";
                break;
        }
        
    }
    let playerInput;
    function startPlay() {
        playerInput = prompt("Rock, paper, or scissors?");
        play(playerInput, computerPlay());
    }
    function play(a,b) {
        if(((a = "rock") && (b = "scissors")) || ((a = "paper") && (b = "rock")) || ((a = "scissors") && (b = "paper"))) {
            alert(`You win, ${a} beats ${b}!`);
        }else if(((a = "rock") && (b = "paper")) || ((a = "paper") && (b = "scissors")) || ((a = "scissors") && (b = "rock"))) {
            alert(`You lose, ${b} beats ${a}`);
        }else if(a == b) {
            alert("It's a tie.");
        }
    }