toUpperCase() 소문자➡️대문자 var sen = 'I Dont THINK that i Like Her'; const sen_uppercase = sen.toUpperCase(); console.log(sen_uppercase); //I DONT THINK THAT I LIKE HER toLowerCase() 대문자➡️소문자 var sen = 'I Dont THINK that i Like Her'; const sen_lowercase = sen.toLowerCase(); console.log(sen_lowercase); //i dont think that i like her