new to JavaScript, simple if function

For Node-RED function nodes you must return a msg object not just a value.

So the correct form for this test is as follows.

if (msg.payload > 25) {
  msg.payload = 1
} else {
  msg.payload = 0;
}

return msg;

You can do it in one line:

return msg.payload > 25 ? 1 : 0;

or if you just need booleans (true/false):

return msg.payload > 25;

Try this:

if (Number(m) > 25){
    return 1;
}
else{
    return 0;
}

You can simplify this a good bit, the else isn't even needed in this case:

if (m > 25) {
  return 1;
}
return 0;

When the return inside the if statement is executed, the code that follows won't be run... making the else statement unnecessary.

The else is redundant when you're just returning like that, and you've got your comparison operators backwards. Right now it'll return 1 if it's less than 25, not greater than.

if (msg.payload > 25) {
return 1;
}
return 0;

You should learn comparision operator where > denotes greater than and < denotes lesser than , you can simplify using ternary operator as

return msg.payload > 25 ? 1 : 0; 

DEMO WITH IF ELSE

function check(m){
if (m > 25){
    return 1;
  }
    else 
    {
      return 0;
    }
};

console.log(check(50));

DEMO WITH TERNARY

function check(m){
  return m > 25 ? 1 : 0; 
};

console.log(check(50));

As you have mentioned that you want to return 1 if value is greater than 25. Please check that you have written wrong condition. Your condition must be this:

if ( m > 25 ){ 
    return 1; 
} else { 
    return 0; 
} 

You are making a very silly mistake of less than and greater than sign. Hope it helps you.

m = msg.payload;
if (m > 25)
{
msg.payload=1;
}
else if (m < 25)
{
msg.payload=0;
}
return msg;