Please check below 2 code snippets the difference is redeclaring as const/let and var in foo function.

for var, it executes but for const and let it throughs error.

Please tell me why it behaves differently.

Below code does not through any error as a is declared as var
var a=12; //Global variable
foo(); //hoisting 
function foo() {
    console.log("a="+a);
    var a=13;
    console.log("a="+a);
    if(true) {
        const a=89;
        console.log("in block="+ a);
    }
    var a=90;    //no error
    console.log("a="+a);    
}

Below code throughs variable declaration error "a has already been declared"

var a=12; //Global variable
foo(); //hoisting 
function foo() {
    console.log("a="+a);
    var a=13;
    console.log("a="+a);
    if(true) {
        const a=89;
        console.log("in block="+ a);
    }
    let a=90;    //Or const a :: error
    console.log("a="+a);    
}