<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<script>
var fan=prompt("投喂的量");
var howfar=prompt("奔跑的距离");
var person={
name="dsm",
age=30,
weight=120,
eat:function(x){
if(x>=0)
{
weight=weight+x;
}
else{alert("喝西北风");}
if(weight<80){alert("瘦到不正常");}
if(weight>=80&&weight<100){alert("偏瘦")}
if(weight>=100&&weight<145){alert("正常");}
if(weight>=145){alert("太胖了");}
return 0;
},
run:function(distance){
if(distance<1000){alert("有点吃力");}
if(distance>=1000&&distance<2000){alert("折磨");}
if(distance>2000){alert("原地去世");}
return 0;
}
}
person.eat(fan);
person.run(howfar);
</script>
</body>
</html>
对象属性名和值之间用冒号，还有未定义weight，帮你改好了：
如有帮助请在我的回答上点个【采纳】，谢谢！
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<script>
var fan=prompt("投喂的量");
var howfar=prompt("奔跑的距离");
var weight;
var person={
name:"dsm",
age:30,
weight:120,
eat:function(x){
if(x>=0)
{
weight=weight+x;
}
else{alert("喝西北风");}
if(weight<80){alert("瘦到不正常");}
if(weight>=80&&weight<100){alert("偏瘦")}
if(weight>=100&&weight<145){alert("正常");}
if(weight>=145){alert("太胖了");}
return 0;
},
run:function(distance){
if(distance<1000){alert("有点吃力");}
if(distance>=1000&&distance<2000){alert("折磨");}
if(distance>2000){alert("原地去世");}
return 0;
}
}
person.eat(fan);
person.run(howfar);
</script>
</body>
</html>