why does console.log only show some of an object code example

Example 1: print object javascript

str = JSON.stringify(obj);
str = JSON.stringify(obj, null, 4); // (Optional) beautiful indented output.
console.log(str); // Logs output to dev tools console.
alert(str); // Displays output using window.alert()

Example 2: console.log object object

const language = this.converstaionLanguage; // {"lang":"en"} <- this is JSON

console.log(language); // [object Object]
console.log(JSON.stringify(language) // {"lang":"en"}