Why do all values change when I change one value in an array of objects? TypeScript

sortDates(){      
    let index = 0;    
    for(let year of this.yearList){          
        let yearInfo = this.yearInfo;
        this.year.push(yearInfo)         
        index++;         
    }
this.year[2].year = '123'

Versus

sortDates(){      
    let index = 0;    
    for(let year of this.yearList){          
        let yearInfo {
        year: '',
        info: [
          {
            date:'Dec',
            total: 0
          },
          {
            date:'Nov',
            total: 0
          },
          {
            date:'Oct',
            total: 0
          }         
        ]
    };
        this.year.push(yearInfo)         
        index++;         
    }

The second one allows me to change the values of 'year' without changing it for all elements. The first changes every element to the same value.

Example: if 2016 was the last value in the for loop, all the values in the array will be 2016 for 'year'

0:{year: "123", info: Array(12)}
1:{year: "123", info: Array(12)}
2:{year: "123", info: Array(12)}

Versus

0:{year: "", info: Array(12)}
1:{year: "", info: Array(12)}
2:{year: "123", info: Array(12)}

Is there a better way to obtain the 2nd result? and why does the first result happen? thanks!

EDIT : Using Object.assign helped