JavaScript static class method is working but non-static method failing the test

I defined Blockchain class:

    class Blockchain {
      constructor() {
        // chain has to be validated all the time. new block should be legit
        this.chain = [Block.getGenesisBlock()];
      }

      addBlock({ data }) {
    const newBlock = Block.mineBlock({ // this function is working properly
      lastBlock: this.chain[this.chain.length - 1],
      data,
    });
    this.chain.push(newBlock);
  }
      // ************ STATIC METHOD IS FAILING TEST *******************
      getLastBlock() {
        return this.chain[this.chain.length - 1];
      }
}

Test case is passing with non-static getLastBlock

it("adds a new block to chain", () => {
    const newBlockData = "datala";
    blockchain.addBlock({ data: newBlockData });
    expect(blockchain.getLastBlock().data).toEqual(newBlockData);
  });

However if I define static method:

static getLastBlock() {
    return this.chain[this.chain.length - 1];
  }

And calling this method with Blockchain class in test file:

 it("adds a new block to chain", () => {
const newBlockData = "datala";
blockchain.addBlock({ data: newBlockData });
expect(Blockchain.getLastBlock().data).toEqual(newBlockData);
});

Gives me this error:

I could not figure out why. I thought I understood everything about this and class in JavaScript.